The effects of nesting site availability on colony size were investigated in a Great Cormorant (Phalacrocorax carbo) colony on dead trees in north-East Italy. Trees have collapsed and colony structure has changed in recent years. After a steady increase in size, the breeding colony decreased dramatically as predicted by estimates of potential nesting space. After the formation of a new sub-colony in living trees, the colony grew again. Tree structures were used differentially, with low- quality sites occupied in years of strong space limitation. However, the proportion of nest sites that were used twice in a season increased only slightly as nesting space diminished, perhaps reflecting the high costs of breeding late. The mean brood size at fledging remained unchanged between the growing and declining phases, suggesting that food availability to cormorants did not change, despite the apparent de crease in fish harvest size. In addition, immature-plumaged cormorants still attempted to breed when the mean nest-site quality was at its lowest. This study provided an example of a breeding colony where yearly changes in size were not a good indicator of its actual phase of development. [KEYWORDS: colony size; Great Cormorant; nest-site selection; nesting space limitation; Phalacrocorax carbo Phalacrocorax-carbo; population]
Original languageEnglish
Pages (from-to)417-423
Issue number3
StatePublished - 1999

ID: 316274